Are you using the Set Item permissions action anywhere within the workflow? If so, make sure you aren't locking the item down in such a way that you no longer have rights to get at it. Also make sure you have permissions on the Task List on which you are doing your approvals/tasks.

Have you tried to wait, say five minutes, before processing the approval task, just in case there is an issue with the list item not being created fully before you attempt to action the task. It sounds like something like this, if you can restart the workflow and the action completes.